The present invention relates to a pulse oximeter for detecting blood oxygenation, and in particular to the elimination of motion artifact which may affect the detected blood oxygenation signal.
Pulse oximeters typically measure and display various blood flow characteristics including but not limited to blood oxygen saturation of hemoglobin in arterial blood, volume of individual blood pulsations and the rate of blood pulsations corresponding to each heartbeat of the patient. The oximeters pass light through human or animal body tissue where blood perfuses the tissue such as a finger, an ear, the nasal septum or the scalp, and photoelectrically sense the change in absorption of light in the tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
The light passed through the tissue is selected to be of one or more wavelengths that is absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light passed through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption.
The optical signal can be degraded by both noise and motion artifact. One source of noise is ambient light which reaches the light detector. Another source of noise would be electromagnetic coupling from other electronic instruments in the area. Motion of the patient can also affect the signal. For instance, when moving, the coupling between the detector and the skin or the emitter and the skin can be affected, such as by the detector moving away from the skin temporarily, for instance. In addition, since blood is a fluid, it may not move at the same speed as the surrounding tissue, thus resulting in a momentary change in volume at the point the oximeter probe is attached.
Such motion can degrade the signal used for making medical decisions, with the clinician being unaware of it. This is especially true if there is remote monitoring of the patient, the motion is too small to be observed, the clinician is watching the instrument or other parts of the patient and not the sensor site, or in a fetus where motion is hidden.
In one oximeter system described in U.S. Pat. No. 5,025,791, an accelerometer is used to detect motion. When motion is detected, readings influenced by motion are either eliminated or indicated as being corrupted. In a typical oximeter, measurements taken at the peaks and valleys of the blood pulse signal are used to calculate the desired characteristic. Motion can cause a false peak, resulting in a measurement having an inaccurate value and one which is recorded at the wrong time. In U.S. Pat. No. 4,802,486, assigned to Nellcor, the disclosure of which is incorporated herein by reference, an EKG signal is monitored and correlated to the oximeter reading to provide synchronization to limit the effect of noise and motion artifact pulses on the oximeter readings. This reduces the chances of the oximeter locking on to a periodic motion signal. Still other systems, such as that set forth in U.S. Pat. No. 5,078,136, assigned to Nellcor, the disclosure of which is incorporated herein by reference, use signal processing in an attempt to limit the effect of noise and motion artifact. The '136 patent, for instance, uses linear interpolation and rate of change techniques to analyze the oximeter signal.
The nature of oximetry readings impose a number of difficulties in dealing with noise. The oximeter relies on mathematical analysis of the readings at two different wavelengths. Because different amounts of light are absorbed at each wavelength, the magnitude of the motion artifact due to the same motion will be different for each signal. This is complicated by the fact that the lights are alternately pulsed, and thus each is influenced by a different amount of motion, since the motion varies with time.
One system, set forth in PCT Publication No. WO 92/15955 (Vital Signals, Inc.) correlates the non-noise portion of two wavelength signals and generates a noise reference signal. The noise reference signal is then provided to an adaptive noise canceler to eliminate the noise from the desired signal.
U.S. Pat. No. 4,714,341 discloses the use of three different wavelengths, rather than two, in order to detect when noise is present. This patent teaches using the first and second wavelength signals to produce a first oxygen saturation value, and then using the first and third wavelength signals to produce a second oxygen saturation value. The two calculated values are then compared. If the values are equal, as they should be absent motion, the signal is presumed to be good. If the values are different, the signal is assumed to contain motion and is disregarded.